Recently, in the field of medical endoscopes, there has been a growing need for optical systems that enable magnified observation in order to perform qualitative diagnosis of lesions.
As an example of such endoscope objective lenses, Patent Literature 1 to Patent Literature 5 disclose lenses that enable magnified observation, having a positive-negative-positive three-group structure in which the second group moves to perform focusing. In addition, Patent Literature 6 discloses lenses having a negative-positive-negative three-group structure in which the second group moves to perform focusing.
In accordance with the need for high-quality diagnostic images, the precision of image-capturing devices such as CCDs and CMOS devices is becoming higher and higher, and with regard to optical systems, there is a demand for not only focusing functions, but also to make them compatible with the increasingly higher precision of image-capturing devices.
Endoscope observation requires a large observation depth from near points to far points, and therefore, optical systems that are almost panfocal, with a large Fno and stopped-down aperture, are required.
On the other hand, the Fno of the optical systems must fall within the following range in order to avoid the effects of diffraction.Fno<2×P/1.22/λ
As image-capturing devices gain higher and higher pixel counts, they are influenced by diffraction, and since the Fno cannot be made very large, the depth becomes small.